In some contexts, we actually have to access the underlying storage of a
node's children (e.g. for proxy nodes). It is important to *not* use
`child()` in this situation, as the child might have been allocated on
the head (e.g. as part of a tree transformation) and thus might
disappear when its parent goes out of scope or is otherwise destroyed.

This patch adds a new function `childStorage()` that works just like
`child()`, but returns the storage (usually a `std::shared_ptr`) of the
child instead. It will fail if the child path is of length zero and the
passed-in object is not pointer-like. If it is pointer-like, the
algorithm assumes that the pointer represents the storage of the root
node and will just return that object.